Nuclear Magnetic Resonance Spectroscopy

Nuclear Magnetic Resonance Spectroscopy
Organic Chemistry Third Edition David Klein Chapter 15 Nuclear Magnetic Resonance Spectroscopy Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.1 Intro to NMR Spectroscopy
Nuclear Magnetic Resonance (NMR) spectroscopy may be the most powerful method of gaining structural information about organic compounds NMR involves an interaction between electromagnetic radiation (light) and the nucleus of an atom We will focus on C and H nuclei for analysis of organic compounds, for obvious reasons The structure (connectivity) of a molecule affects how the radiation interacts with each nucleus in the molecule Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

Protons and neutrons in a nucleus behave as if they are spinning If an atom has an odd number of protons and/or odd number of neutrons, it will have net nuclear spin Examples: The spinning charge in the nucleus creates a magnetic moment Magnetic moment = magnetic field Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

If the normally disordered magnetic moments of atoms are exposed to an external magnetic field (B0), their magnetic moments will align with the field, or against the field: Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

When an atom with an α spin state is exposed to radio waves with just the right energy (DE), the photons are absorbed and spin flips to the β spin state The stronger the magnetic field, the greater the energy gap Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

B0 causes electrons to circulate, inducing an opposing magnetic field (diamagnetism) Nuclei then experience less B0, and are said to be shielded, requiring less energy to spin flip Nuclei surrounded by less electron density are said to be deshielded Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

The amount of radio wave energy necessary for the α  β energy transition depends on the electronic environment for the atom When the α spins are flipped to β spins, energy is absorbed, and the atoms are said to be in resonance The use of the term, “resonance” here is totally different from when we are talking about electrons in molecular orbitals The amount of energy required for each 1H to spin flip tells us about it’s electronic environment, and thus the structure of the compound. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.2 Acquiring a 1H NMR Spectrum
NMR requires a strong magnetic field (B0) and radio wave energy The strength of the magnetic field affects the energy gap Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

The strong magnetic field is created when a high current is passed through a superconducting material at extremely low temperature (≈4 Kelvin) The greater the current, the greater the magnetic field Pulsed Fourier-transform spectrometer: a brief pulse of radio energy (all relevant wavelengths) is used to excite the sample Each of the atoms is excited and then relaxes, emitting energy The emitted energy is recorded as a free induction decay (FID) Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

The FID contains all of the information for each atom A mathematical treatment called a Fourier-transform separates the signals so an individual signal can be observed for each atom that was excited Often multiple FIDs are taken and averaged together Before analysis, NMR samples must be prepared neat or in a liquid solution (deuterated solvent) and placed in a small NMR tube The sample is placed into the magnetic field and the tube is spun at a high rate to average magnetic field variations or tube imperfections Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

Deuterated solvents must be used (no H’s) the sample is generally at room temp when collecting the HNMR spectrum Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.3 Characteristics of a 1H NMR Spectrum
NMR spectra contain a lot of structural information Number of signals Signal location (chemical shift) Signal intensity (area under the signal) Signal shape (splitting pattern) Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals The number of signals in a 1H NMR spectrum indicates the number of different kinds of protons in a compound Identical protons are chemically equivalent. Such protons fall into the category of being either homotopic or enantiotopic Homotopic protons: if the molecule has an axis of rotational symmetry that allows one proton to be rotated onto the other without changing the molecule Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals Another test for homotopic protons is to replace the protons one at a time with another atom If the resulting compounds are identical, then the protons that you replaced are homotopic Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals Enantiotopic protons: if the molecule has a plane of reflection that makes one proton the mirror image of the other Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals The replacement test is universal
It will work to identify any equivalents protons whether they are homotopic or enantiotopic If the resulting compounds are enantiomers, then the protons that you replaced are enantiotopic Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals To determine if protons are equivalent, look for rotational symmetry. If none is found, then look for a plane of symmetry Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals The replacement test produces diastereomers:
These protons are called diastereotopic, and not chemically equivalent Practice with SkillBuilder 15.1 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals If the protons are neither homotopic nor enantiotopic, then the are NOT chemically equivalent Consider the indicated H’s below, in 2-butanol: These protons cannot be interchanged by rotational symmetry or reflectional symmetry Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals There are some shortcuts you can take to identify how many signals you should see in the 1H NMR The 2 protons on a CH2 group will be equivalent if there are NO chirality centers in the molecule The 2 protons on a CH2 group will NOT be equivalent if there is a chirality center in the molecule Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Number of Signals There are some shortcuts you can take to identify how many signals you should see in the 1H NMR The 3 protons on any methyl group will always be equivalent to each other Multiple protons are equivalent if they can be interchanged through either a rotation or mirror plane Practice with SkillBuilder 15.2 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Variable Temperature NMR
Recall that cyclohexane chairs have 6 equatorial and 6 axial protons the axial and equitorial protons have different electronic environments Technically, there are two types of protons here, but only one signal is observed in the 1H NMR Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.4 Variable Temperature NMR
At room temperature, the chair interconversion occurs rapidly The NMR is not fast enough to see the individual structures, so the average is observed (one signal) If the temperature was reduced to -100 ˚C, ring flipping is slow enough to observe two signals Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift In many NMR solvents, 1% TMS is added as an internal standard. The frequency of the protons in TMS is lower than that observed for most organic compounds The shift for a proton signal is calculated as a comparison to TMS For benzene on a 300 MHz instrument Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift The shift for a proton signal is calculated as a comparison to TMS For benzene on a 60 MHz instrument The Hz of the signal is different in different instruments, but the shift relative to TMS (δ) is constant Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift The shift for a proton signal is calculated as a comparison to TMS The shift relative to TMS (δ) is a dimensionless number, because the Hz units cancel out Units for δ are often given as ppm (parts per million), which simply indicates that signals are reported as a fraction of the operating frequency of the spectrometer Most 1H signals appear between 0-10 ppm Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift Early NMRs analyzed samples at a constant energy over a range of magnetic field strengths low field strength = downfield high field strength = upfield Shielded protons required a stronger external magnetic field to be excited at the same energy as deshielded protons. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift Higher Energy Lower Energy
Current NMRs analyze samples at a constant field strength over a range of energies Shielded protons have a smaller magnetic force acting on them, so they have smaller energy gaps and absorb lower energy radio waves Higher Energy Lower Energy Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift Alkane protons generally give signals around 1-2 ppm Protons can be shifted downfield when nearby electronegative atoms cause deshielding. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift To predict chemical shifts, start with the standard ppm for the type of proton (methyl, methylene, or methine) Use table 16.1 to adjust the ppm depending on proximity to certain function groups Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift Handbooks, or tables found online at credible websites, can be used for functional groups beyond table 15.1 Practice with SkillBuilder 15.3 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift – Anisotropic Effects
When the electrons in a pi system are subjected to an external magnetic field, they circulate and produce a magnetic field, causing diamagnetic anisotropy Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift Diamagnetic anisotropy means that different regions of localized space has different magnetic strengths Regions outside the ring have increased magnetic field Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift The result of the diamagnetic anisotropy effect is similar to deshielding for aromatic protons. More magnetic field means more energy required to spin-flip the proton(s) Aromatic protons appear in the neighborhood of 7 ppm. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift The result of the diamagnetic anisotropy effect is similar to shielding for protons on the inside of the pi system protons outside the ring: appear at 8 ppm Protons inside the ring: appear at -1 ppm. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.5 Chemical Shift It is necessary to be familiar with the ppm values of these common types of protons Practice with Conceptual Checkpoint 15.10 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.6 Integration The integration (area) under the peak quantifies the relative number of protons giving rise to a signal A computer will calculate the area of each peak representing that area with a step-curve The curve height represents the integration Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.6 Integration The computer operator sets one of the peaks to a whole number to let it represent a number of protons The computer uses the integration ratios to set the values for the other peaks 1.48 1.56 1.00 1.05 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.6 Integration Integrations represent numbers of protons, so you must adjust the values to whole numbers If the integration of the first peak is doubled, the computer will adjust the others according to the ratio 2.96 3.12 2.00 2.10 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.6 Integration The integrations are relative quantities rather than an absolute count of the number of protons Symmetry can also affect integrations 3-pentanone has two kinds of protons. The relative integration value of the two signals would be 2:3 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.6 Integration The molecular formula (C5H10O) must be known in order to know that the absolute ratio is actually 4:6, and not 2:3 Practice with SkillBuilder 15.4 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Multiplicity results from magnetic affects that protons have on each other Consider protons Ha and Hb Since protons align with or against the external magnetic field, so there will be two different magnetic environments for Ha : Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Consider an example where there are two protons on the adjacent carbon Here, there are three possible environments for Ha Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Table 15.3 shows how the multiplicity trend continues By analyzing the splitting pattern of a signal in the 1H NMR, you can determine the number of equivalent protons on adjacent carbons Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity The trend in table 15.3 also allows us to predict splitting patterns When there are “n” number of neighboring protons that are chemically equivalent, there will be how the n+1 peaks in the signal Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Remember three key rules for splitting:
Equivalent protons can not split one another because they resonate together To split each other, protons must be within a 2 or 3 bond distance Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Remember three key rules
The n+1 rule only applies to protons that are all equivalent The splitting pattern observed for the proton shown below will be more complex than a simple triplet Complex splitting will be discussed later in this section Practice with SkillBuilder 15.5 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity The degree to which a neighboring proton will couple to its neighbor is called a coupling constant, or J value The J value is the distance between peaks of a splitting pattern measured in units of Hz protons splitting each other will have the same coupling constant Jab = Jba Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Higher field strength instruments will give better resolution between peaks, because the coupling constant is a smaller percentage of the overall Hz available. 300 MHz spectrometer avoids overlapping of peaks (right), compared to the 60 MHz spectrometer (left) Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Sometimes recognizable splitting patterns will stand out in a spectrum An isolated ethyl group gives a triplet and a quartet The triplet and quartet must have the same coupling constant if they are splitting each other Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity A peak with an integration equal to 9 suggests the presence of a tert-butyl group An isolated isopropyl group gives a doublet and a septet Practice with conceptual checkpoint 15.17 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Complex splitting results when a proton has two different kinds of neighboring protons If Jab is much greater than Jbc then the signal will appear as a quartet of triplets In the molecule shown, Hb is split into a quartet by Ha and into a triplet by Hc Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Complex splitting results when a proton has two different kinds of neighboring protons If Jbc is much greater than Jab, the signal will appear as a triplet of quartets If Jbc is similar to Jab, then several lines may overlap, and the signal is called a multiplet Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity Splitting is not observed for some protons. Consider ethanol The protons bonded to carbon split each other, but the hydroxyl proton is not split Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.7 Multiplicity The hydroxyl proton and other labile or exchangeable protons undergo rapid exchange with trace amounts of acid. Show a reasonable mechanism Such exchange blurs the shielding/deshielding affect of the neighboring protons giving a singlet that is often broadened Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.9 Using 1H NMR to Distinguish between Compounds
The three molecules below might be difficult to distinguish by IR of MS. They are easily distinguished with 1H NMR because they do not have the same # of protons. Practice with SkillBuilder 15.7 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.10 Analyzing a 1H NMR Spectrum
With a given formula and 1H NMR spectrum, you can determine a molecule’s structure by a 5-step process Calculate the degree of unsaturation or hydrogen deficiency index (HDI). Consider the number of NMR signals and integration (gives clues about the symmetry in the molecule) Analyze each signal, and draw fragments consistent with each signal Assemble the fragments into a complete structure Verify the proposed structure is consistent with all of the spectral data Practice with SkillBuilder 15.8 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.11 Acquiring a 13C NMR Spectrum
Because 1H is by far the most abundant isotope of hydrogen, 1H NMR signals are generally strong 13C only accounts for about 1% of carbon atoms in nature, so a sensitive receiver coil and/or concentrated NMR sample is needed In 1H NMR, shift, splitting, and integration are important In 13C NMR, only the number of signals and the chemical shift will be considered Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.11 Acquiring a 13C NMR Spectrum
In 13C NMR, the 1H-13C splitting is often so complex that the spectrum is unreadable To elucidate the 13C spectrum and make it easier to determine the total number of 13C signals, 13C NMR are generally decoupled In the vast majority of 13C spectra, all of the signals are singlets Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.12 Chemical Shifts in 13C NMR
Compared to 1H, 13C atoms require a different frequency of energy to excite (resonate) Compared to the standard TMS, 13C NMR signals generally appear between 220 and 0 ppm Each signal in a 13C NMR spectrum represents a unique type of carbon Planes and axes of symmetry can cause carbon atoms to be electronically equivalent Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

Like 1H signals, chemical shifts for 13C signals are affected by shielding or deshielding Practice with SkillBuilder 15.9 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.13 DEPT 13C NMR Spectroscopy
13C spectra generally give singlets that do not provide information about the number of hydrogen atoms attached to each carbon Distortionless Enhancement by Polarization Transfer (DEPT) 13C NMR provides information the number of hydrogen atoms attached to each carbon DEPT-90: Only CH signals appear DEPT-135: CH3 and CH give (+) signals, and CH2 give (-) signals Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.13 DEPT 13C NMR Spectroscopy
DEPT-90: Only CH signals appear DEPT-135: CH3 and CH = (+) signals, CH2 = (-) signals Practice with SkillBuilder 15.10 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

15.13 Magnetic Resonance Imaging (MRI)
MRI (Magnetic Resonance Imaging) instruments are essentially 1H NMR spectrometers The body is analyzed rather than a sample in an NMR tube Different tissues have different concentrations of protons. The MRI gives a 3D image of different tissues. Radio waves and magnetic field are very mild, so no side effects are experienced Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 3e

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